Ailing knee knocks defending champ Wawrinka out of US Open

Stan Wawrinka chose long-term health over a US Open title defense. The defending champion pulled out of the tournament Friday because of an injured knee, saying he decided to undergo a "medical intervention" and sit out the rest of 2017. He did not provide any specifics about the injury or treatment.

"This was the only solution to make sure I will be able to compete at the top level for many more years," Wawrinka said in a statement through his agency.

He joins the man he beat in last year's final at Flushing Meadows, Novak Djokovic, in calling it quits for this season because of injury.

Like Roger Federer last year, Wawrinka couldn't make it to New York for the season's final major, which begins Aug. 28.

The fourth-ranked Wawrinka added last year's title to his 2014 Australian Open and 2015 French Open championships. He got back to the finals in Paris in June, losing to Rafael Nadal, but was eliminated by Daniil Medvedev in the first round at Wimbledon.

The 32-year-old Swiss was clearly bothered by his left knee then, icing it during changeovers. He said after the defeat the knee had been a problem on and off all season and he needed to figure out what was wrong.

Wawrinka finishes with a 26-11 record with just one title this season. He says he is looking toward his recovery and playing in 2018.

"I love this sport and I will work hard to get back to my top level and play many more years," Wawrinka said.

Wawrinka follows his former Olympic doubles and Davis Cup champion teammate in having to cut short his season because of his knee. Federer sat out the second half of 2016 but has returned strong this year, winning two majors and beating Wawrinka in the finals at Indian Wells.

Djokovic decided late last month he would miss the US Open because of an injured right elbow, ending his streak of playing in 51 consecutive Grand Slam tourna- ments. He aims to return to the ATP Tour in January.

Anderson upsets topseeded Thiem at Citi Open

Kevin Anderson survived a tense third-set tiebreaker to upset topseeded Dominic Thiem and advance to the quarterfinals of the Citi Open.

The 6-foot-8 South African veteran fought off a match point before serving out the tiebreaker to win 6-3, 6-7 (6), 7-6 (7) on Thursday night and improve his career record to 6-0 against the seventhranked Thiem. The match lasted 2 hours, 46 minutes.

The Citi Open is a hard-court tuneup for the US Open, which starts July 28. Anderson's three career titles have all come on hard courts. Thiem has three titles this season, two on clay, and he reached the semifinals at the French Open.

Third-seeded Milos Raonic, the 2014 Citi Open champion, won a lengthy first-set tiebreak on the way to beating Marcos Baghdatis 7-6 (7), 6-3, and eighth-seeded Jack Sock beat fellow American Jared Donaldson 7-6 (6), 6-2. Raonic and Sock will face each other in the next round.

In the women's draw, Canadian wild card Bianca Andreescu knocked off second-seeded Kristina Mladenovic in straight sets to advance to the quarterfinals. Andreescu beat Mladenovic 6-2, 6-3 on Thursday. In other women's matches, top-seeded Simona Halep came back from losing the first set to beat Mariana Duque-Marino 3-6, 6-4, 6-2, and Andrea Petkovic beat Eugenie Bouchard 6-2, 4-6, 6-0.

Five of the top seven seeds in the women's draw reached the quarterfinals.

Davis Cup singles matches to remain best of 5 sets

Davis Cup singles matches will remain best-of-five sets after a proposal for them to be played as best-of-three tiebreak sets was re- jected at an annual general meeting of the International Tennis Federation on Friday.

The motion received 63.54 percent support from member nations, but reforms require a two-thirds majority in the voting.

ITF President David Haggerty says: "We respect the decision of the AGM but are disappointed that our member nations have not approved the full package of Davis Cup and Fed Cup reforms endorsed by the ITF Board."

The ITF agreed that finalists in both Davis Cup and Fed Cup will be guaranteed the choice of hosting their first-round tie the following year.

Kohlschreiber, Sousa advance to Generali Open final

Philipp Kohlschreiber and Joao Sousa advanced to the final of the Generali Open on Friday, with both players seeking their first title of the season.

Sousa ended the run of Austrian wild card Sebastian Ofner, who appeared in his maiden semifinal, with a 7-6 (4), 7-6 (4) win. The 62nd-ranked Portuguese will seek his third career title and first since winning in Valencia two years ago.

Earlier, Kohlschreiber held serve throughout against second-seeded Fabio Fognini and defeated the 25th-ranked Italian 7-5, 6-3. Fognini missed out on his second final in as many weeks after winning his fifth career title in Gstaad, Switzerland, last week.

The 47th-ranked Kohlschreiber is 1-2 against Sousa but won their only previous match this season. Both players lost their other final this year, Kohlschreiber to Borna Coric in Marrakech in April and Sousa to Jack Sock in Auckland in January.

Kohlschreiber will appear in his third final at the clay-court event in the Austrian Alps. In 2012 the German was beaten by Robin Haase of the Netherlands in the final, while in 2015 he defeated Paul-Henri Mathieu of France for the title.